HomeMy WebLinkAbout01.9 Geological & Soils Report•
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Preliminary Geotechnical Investigation
Eshelman Property
Garfield County, Colorado
Project No. 29-132
August 28, 2009
Prepared for:
Mr. Roger Eshelman
7378 County Road 100
Carbondale, Colorado 81623
Prepared by:
Yeh and Associates, Inc.
170 Mel Ray Road
Glenwood Springs, Colorado 81601
Phone: 970-384-1500
Fax: 970-384-1501
Eshelman Property
Project No. 29-132
Table of Contents
0 Page
PURPOSE AND SCOPE OF STUDY 1
PROPOSED CONSTRUCTION 1
SITE CONDITIONS 1
SITE GEOLOGY 2
GEOLOGIC HAZARDS 2
SUBSURFACE CONDITIONS 3
SITE DEVELOPMENT 4
FOUNDATION CONDITIONS 5
SLABS -ON -GRADE 5
SUBSURFACE DRAINAGE 5
SURFACE DRAINAGE 6
• WATER SOLUBLE SULFATES 6
LIMITATIONS 6
figures No.
Approximate Test Pit Location 1
Test Pit Log and Legend 2
Sieve Analysis Results 3, 4
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Eshelman Property Project No. 29-132
The parcel is located at an approximate elevation of 7100 feet in a rural area. The site
was steeply sloping up to the northeast and east at an estimated grade of 30 percent.
Vegetation and surface conditions at the site included native shrubs, scrub oak, small trees,
grasses and small boulders.
SITE GEOLOGY
The site was located in a unique and complex geologic setting. The project area was on
the north side of the Roaring Fork River valley in an area known as Missouri Heights and,
geologically, in the western half of an area known as the Carbondale Collapse Center. This
area defines a regional collapse structure due to evaporite, or salt, dissolution in the subsurface
and includes features such as sinkholes, subsidence troughs and collapse debris. Collapse
debris was documented at lower elevations within one-half mile of the site.
Based on information shown on the Geologic Map of the Carbondale Quadrangle,
Garfield County, Colorado (1997), the project area was situated on colluvium overlying Tertiary
age basalt. Basalt boulders were noted on the property. A small landslide was mapped
adjacent to the southeast edge of the property. Approximately one-third mile south and three-
fourths mile west of the property were areas mapped as collapse debris, which is defined as
deposits of deformed bedrock and surficial deposits formed in response to vertical collapse of
underlying evaporite/salt deposits. Surficial deposits at the site include colluvium made up of
silty sand and gravels with basalt boulders. These deposits thin upslope to the east where the
basalt boulders covered the surface. At depth, the area is most likely underlain by the Eagle
Valley Evaporite.
The predominate soil at the site was shown in the Soil Survey Aspen -Gypsum Area,
Colorado, 1981, as Morval-Tridell complex, which is deep, well to excessively drained soil
derived from basalt. In this area, the colluvium was best described as the Tridell soil that is a
stony to cobbly, sandy loam to stony loamy sand.
GEOLOGIC HAZARDS
Based on our investigation, the Garfield County hazard maps for soil and geology did not
show specific geohazards at this site. However, potential sinkhole locations and depths are
highly unpredictable. Based on our observations, the collapse debris was mapped at lower
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Eshelman Property Project No. 29-132
elevations than Lot 1. Based on the performance of existing structures (up to 25 years,
approximately), development on Lot 1 should exhibit similar performance. To reduce the
likelihood of sinkhole or collapse development, drainage should be carefully planned to divert
surface flows away from structures and improvements. We believe the risk of sinkhole
development cannot be eliminated or mitigated.
We did not observe evidence of slope instability at or above the subject site. Based on
the soil survey documents, soil at this site may experience rapid runoff and may be subject to
water erosion. Minor slope creep and erosion is likely to occur. However, cut slopes planned in
the existing slope could induce instability. We recommend cut slopes planned on existing
slopes with grades in excess of 30 percent be evaluated by the geotechnical engineer. The
existing cut slopes behind the barn and house on Lot 2 were about 10 to 20 feet in height and
were near vertical. We observed 6 to 12 -inch sized rocks within the cut slopes. Larger boulders
up to 4 -foot in diameter were observed behind the existing residence. Based on our
observations, the existing cut slopes are over steepened. The over steepened slopes are at an
increased risk for rockfall and erosion.
SUBSURFACE CONDITIONS
To investigate subsurface conditions, one test pit was excavated on August 5, 2009 at
the approximate location presented on Figurel. The test pit location was selected by the client
and excavated to a depth of 10 feet (from the original grade from the cut side on the existing
bench) with a rubber tire backhoe. Soil samples were collected at specified depths. The
collected samples were transported to our laboratory where they were examined and classified.
Laboratory tests included moisture content, grain size analysis, Atterberg limit testing and water-
soluble sulfates.
Generally, the subsoils encountered in the test pit consisted of varying amounts of poorly
graded to silty gravel and poorly graded to silty sand. The entire depth of the test pit was
interlayered with cobbles and boulders. The bottom of the test pit was terminated at
approximately 10 feet, where cobbles and boulders were very larger and digging became more
difficult. Bedrock was not encountered.
One gravel sample tested had 7 percent fines (passing No. 200 sieve). Atterberg limit
testing indicated a liquid limit of 46 percent and plastic index of 14 percent. One sand sample
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Eshelman Property Project No. 29-132
tested had 10 percent fines and was non -liquid and non -plastic. Water soluble sulfate testing at
4 feet indicated no water soluble sulfates were present. The gravel classified as a GP -GM and
the sand as a SP -SM, according to the Unified Soil Classification System (USCS). The
laboratory test results are presented in Figures 3 and 4 and are summarized in the Summary of
Laboratory Test Results table.
Groundwater was not encountered during this investigation and the subsoils were
slightly moist. Variations in groundwater conditions may occur seasonally. The magnitude of
the variation will be largely dependent upon the amount of spring snowmelt, local landscape
irrigation practices, the duration and intensity of precipitation, site grading changes, and the
surface and subsurface drainage characteristics of the surrounding area. Perched water tables
may be present, but were not encountered in the test pit.
SITE DEVELOPMENT
Based on conversations with the client, the actual building envelope/footprint for Lot 1
will be situated above the test pit location. Grading/site plans were not provided at the time of
our investigation. We anticipate minor cuts and fills. Based on our test pit and visual
observations, excavation of the cobbles and boulders should be possible using typical heavy
duty equipment, but may require heavy ripping and/or blasting with depth. We recommend cut
slopes planned at the toe or on existing slopes with grades in excess of 30 percent be evaluated
by the geotechnical engineer.
Areas to receive fill should be stripped of vegetation, organic soils and debris. Topsoil is
not recommended for fill material. Fill should be placed in thin, loose lifts of 8 inches thick or
Tess. For silt and clay soils, we recommend these materials be moisture conditioned to 0 to 3
percent above optimum moisture content and compacted to at least 95 percent of maximum
standard Proctor dry density (ASTM D 698). For granular soils (sand and gravel), we
recommend the materials be moisture conditioned to within 2 percent of optimum moisture
content and compacted to the specification above. Placement and compaction of fill should be
observed and tested by a representative of the geotechnical engineer.
We recommend that permanent soil cut slopes be constructed at a minimum slope of
3H:1 V. Fill slopes should be constructed no steeper than 3H:1 V. Surface water should be
directed away from the crest of slopes. The slopes should be protected from erosion by
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Eshelman Property Project No. 29-132
revegetation or other means. The risk of slope instability increases if seepage is encountered in
cut slopes. Flatter slopes may be required if significant seepage is encountered in cut slopes.
FOUNDATION CONDITIONS
Eagle Valley Evaporite bedrock is located below the entire site. We believe there is a
potential for sinkhole develop throughout the site. We believe the risk of sinkhole development
cannot be eliminated or mitigated. Potential sinkhole locations are difficult to predict. However,
based on our observations, we believe the potential risk is low at this site.
We believe the proposed residence on Lot 1 could be supported on a footing foundation
placed on the silty sand, gravel or properly compacted fill. We anticipate maximum allowable
soil pressures for this site will likely range from 2,000 to 3,000 psf. If the owner is willing to
accept the risk of potential settlement due to sinkholes, a design level investigation should be
performed after the specific house location is chosen and grading is completed to provide a
design level foundation recommendation.
SLABS -ON -GRADE
• Based on our investigation, we believe the near surface soils are suitable for lightly
loaded slabs. Slab -on -grade construction should be feasible on the silty sand and gravel soils.
Slab -on -grade garage and basement floors are susceptible to settlement due to potential
sinkholes. Similar to footing foundations, we believe there could be a risk of poor performance
due to collapse resulting in sinkhole development. To reduce movement related damage, slabs
should be separated from all load bearing walls and columns with expansion joints that allow
vertical movement. Control joints should be used to reduce damage from shrinkage cracking.
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SUBSURFACE DRAINAGE
Groundwater was not encountered during our investigation. Surface water typically
flows through permeable wall backfill and collects at the backfill and natural soil interface
resulting in saturated foundation soils and/or wet crawlspace and basement conditions. To
reduce water accumulation outside foundation walls and reduce moist crawlspace and
basement conditions, a foundation drain should be installed around the exterior of the
foundation walls, Drains can be installed on the interior of crawlspace areas after completion of
construction, if groundwater develops.
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Eshelman Property Project No. 29-132
SURFACE DRAINAGE
Surface drainage is crucial to the performance of foundations, flatwork and slopes.
Water infiltration into slopes can reduce slope stability. Concentrated surface runoff should not
be allowed to flow down slopes. Water should not be allowed to pond at the top of slopes.
Areas where ponding of water occurs would increase the risk of settlement or the development
of sinkholes.
We recommend the ground surface surrounding the residence be sloped to drain away
from the structure. Backfill around foundations should be moisture conditioned and compacted.
Roof downspouts and drains should discharge beyond the limits of the backfill. Failure to follow
these recommendations could result in collapse of the subsoils resulting in foundation and/or
slab -on -grade settlement.
WATER SOLUBLE SULFATES
Based laboratory test results, we anticipate a Class 0 exposure for concrete due to the
presence of water-soluble sulfate. Based on ACI 201.2R-01, "Guide to Durable Concrete,"
concentrations between 0 and Tess than 0.1 percent represent Class 0 exposures (negligible).
For cast -in-place structures placed on natural soils, ACI requires no special requirements for
sulfate resistance.
LIMITATIONS
The analyses and recommendations presented in this report are based upon our data
obtained from the test pit at the indicated location, field observations, laboratory testing, our
understanding of the proposed construction and other information discussed in this report. It is
possible that subsurface conditions may vary between or beyond the points explored. The
nature and extent of such variations may not become evident until construction. If variations
appear, we should be contacted immediately so we can review our report in light of the
variations and provide supplemental recommendations as necessary. We should also review
the report if the scope of the proposed construction, including the proposed Toads, finished
elevations or structure locations, change from those described in this report. The conclusions
and recommendations contained in this report shall not be considered valid unless Yeh and
Associates reviews the changes and either verifies or modifies the conclusions of this report in
writing.
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LEGEND:
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1 P- 1 Indicates approximate location
MIN of exploratory test pit
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100
SCALE: 1" = 100'
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*0-190
Approximate
Test Pit Location
Project No. 29-132 Figure 1
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TP -1
Legend
Sample Types
Bulk sample was obtained from test pit at the depths indicated.
5 Soil Lithology
Gravel, poorly graded with silt, interlayered with cobbles and small
boulders, medium dense to dense, slightly moist to moist, tan,
white, brown (GP -GM).
Sand, poorly graded with silt, interlayered with gravels, cobbles
and small boulders, medium dense to dense, slightly moist to
moist, brown, dark brown (SP -SM).
Notes:
1. Test pit was excavated on August 5, 2009
using a rubber tire backhoe.
2. Test pit description is subject to explanations
contained in this report.
3. Groundwater was not encountered during this
investigation.
�A
YEH AND ASSOCIATES, INC.
GEOTECHNICAL ENGINEERING CONSULTANTS
Eshelman Property
Project Number: 29-132 Figure 2
• ercent Passin•
el
100
90
80
70
60
50
40
30
20
10 -
0
Sieve Analysis
Sieve Opening in Inches
12" 6" 3" 2" 1" 3/4" 1/2" 3/8"
1 I I
T T
4
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U.S. Standard Sieves
8 10 16 30 40 50
100
200
Hydrometer Analysis
Size of Particles in mm
i
I
r
r
7r ---
I
1
f
i
I
1
1
r 1
1
I
I
I
1
I
1
1111111111.1116
1
I
100 10 1
Particle Size (mm)
Gravel (%)
Sand (%)
77
16
LL
PL
46
32
Project Name: Eshelman Property
Sample ID: TP -1
0.1
0.01
Sieve
Size
3"
2 V2"
2"
1 1/2.
1"
Passing
28
28
28
28
28
3�4 u
1/2.
#4
#10
#40
#200
28
27
26
23
19
14
7
Fines (%)
Sample
Description:
7
PI
14
Sample
Depth (ft.):
GRAVEL with silt and sand (GP -GM)
4
Yeh & Associates, Inc.
Geotechnical Engineering Consultants
SIEVE ANALYSIS
Drawn By: SW
Checked By: RDJ
Date: 08/27/09
Project No.:
Figure No.:
29-132
3
Revised 04/227/2004
Percent Passing
100
90
90
70
60
50
40
30
20
10
0
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12"
I
6"
2" 1" 3/4" 1/2" 3/8"
4
8 10 16 30 40 50
100 200
100 10
Particle Size (mm)
0.1
0.01
Sieve
Size
Sieve Analysis
Hydrometer Analysis
Sieve Opening in Inches
U.S. Standard Sieves
Size of Particles in mm
12"
I
6"
2" 1" 3/4" 1/2" 3/8"
4
8 10 16 30 40 50
100 200
100 10
Particle Size (mm)
0.1
0.01
Sieve
Size
Passing
3"
-
2 1/2"
-
2"
-
1 1/2"
-
1"
-
3/4"
83
1/2,,
83
3/$"
82
#4
72
#10
50
#40
25
#200
10
Gravel (%)
28
LL
Project Name: Eshelman Property
Sand (%)
62
PL
Sample ID: TP -1
irAYeh & Associates, Inc.
Geotechnical Engineering Consultants
Fines (%)
10
PI
Sample
Depth (ft.):
8
SIEVE ANALYSIS
Sample
Description:
SAND with silt and grave (SP -SM)
Drawn By: SW
Checked By: RDJ
Date: 08/27/09
Project No.:
29-132
Figure No.:
4
Revised 04/227/2004
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Ell YEH & ASSOCIATES, INC
Project No:
29-132
Summary of Laboratory Test Results
Project Name:
Eshelman Property
- Indicates non-liqu
NP - Indicates non -plastic
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Sample Location
Natural
Water
Soluble
Sulfates
(%)
Gradation
Atterberg Limits
Test Pit
No.
Depth (ft)
Sample
Type
Moisture
Content
(%)
Gravel
> #4
(%)
Sand
0
(/°)
Fines <
#200
(%)
LL
PL
PI
Soil Description
TP -1
4
Bulk
10.7
0.000
77
16
7
46
32
14
Gravel, sandy, slightly silty (GP -GM)
8
Bulk
18.8-
28
62
10
NL
NP
NP
Sand, slightly silty (SP -SM)
- Indicates non-liqu
NP - Indicates non -plastic
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